The present invention relates to a coating for elastomeric linear profiles, where the coating comprises solid lubricant embedded into a polymeric matrix. It further relates to a wiper blade for windshield wipers, comprising this coating, and also to a process for producing an elastomeric linear profile coated by the invention.
Wiper blades for windshield wipers can generally adapt to the shape of the windshield of the motor vehicle, and remain adequately flexible at various temperatures. Wiper blades are therefore generally produced from elastomer profiles and rubber materials, for example natural rubber or chloroprene. There can also be wiper blades manufactured from materials such as silicone rubber or polyurethane rubber.
Elastomers have higher coefficients of sliding friction than other materials such as glass or plastic. In the case of a wiper blade of a windshield wiper, the force that has to be exerted for a horizontal movement of the windshield wiper along the windshield is many times the pressure force exerted vertically through the contact of the windshield with the profile of the wiper blade. In order to reduce friction, the wiper blades can be provided with coatings which have a low coefficient of friction.
Wiper blades are often used on windshields which have been equipped with hydrophobic properties. The behavior of wiper blades on hydrophobic windshields is similar to the slip behavior on a dry surface. The water-repellent surface of the windshield prevents formation of any friction-reducing water film between the surface and the wiper blade. Any additional coating for wiper blades therefore has to comply with the requirements in relation to frictional properties, wear resistance, and smear-free wiping.
Conventional coatings for wiper blades include graphite or molybdenum disulfide as particulate solid lubricant, in order to reduce the coefficient of friction of the wiper blade on the windshield surface. The effectiveness of graphite as solid lubricant mostly depends on the relative concentration thereof in the coating.
A disadvantage of known coatings using particulate graphite as lubricant is that the softness and the required high relative proportion of the graphite particles reduces coherence within the coating. The coating is therefore subject to increased wear during service. The proportion of graphite in the coating is generally set higher than the critical pigment volume concentration, in order to achieve the low coefficient of friction demanded. Cohesion forces are thus reduced, and the coating becomes less robust.
The use of hard solid lubricants to increase wear resistance leads to poorer wiping quality caused by smearing and streaking. If the proportion of graphite is reduced, for example to below 30%, the result is that the surface of each graphite particle is mainly coated with the binder. This reduces the lubricant effect of the graphite and thus increases the coefficient of friction of the coating.
US 2003/0087767 A1 discloses a rubber wiper blade with a coating film on both surfaces of the lip portion, where the coating film comprises a particulate solid lubricant and a binder. After the drying or hardening process, the 0.5% modulus of the binder is 1 MPa or more and its elongation at break is 1% or more. The binders of said publication are in polymeric form from the outset here. For the coating process, they are dissolved in solvents and applied to a wiper blade. After any preceding crosslinking process by means of curing agents, the solvent is removed and the finished coating is formed. However, said specification does not give any detail of the abrasion resistance of the coating and therefore the lifetime of the wiper blade. When predominantly organic solvents have to be used for applying the binder and the solid lubricant to the wiper blade, this is economically disadvantageous. Predominant use of organic solvents is moreover undesirable for environmental reasons.